Transparent, biaxially oriented polyester films featuring a reduced level of mechanical properties (lower strengths, lower moduli) are known in the prior art. In most cases, the films acquire their altered properties via use of aromatic polyesters other than standard polyethylene terephthalate. Examples of these are the widely used glycol-modified aromatic polyesters, e.g. polybutylene terephthalate (PBT) or polytrimethylene terephthalate (PTT). As an alternative to this, there are known aromatic polyesters whose modification relates to the acid component: polyethylene isophthalate (IPA).
An example here is provided by JP 2000094473, which describes use of a polyester film with greater than or equal to 80% by weight of PTT, and which achieves a modulus of elasticity of less than or equal to 5884 N/mm2. The film features good formability and can by way of example be used as transfer film. The disadvantage of said film is that the raw materials used do not permit achievement of the desired low mechanical strengths.
JP 2004202702 describes a further example. An aim of the invention is to produce a film that tears easily. This is achieved firstly via use of polyesters (blends and copolymers) in the base layer of the film whose melting points are lower by at least 10° C. than the melting points of the polyesters in the outer layers, and secondly via selection of certain parameters in heat-setting. Again, said films, the base layer of which is produced in the examples from blends or, respectively, copolymers of PET with IPA or PBT, cannot achieve the desired low mechanical strength.
EP-A-0 415 383 provides a further example here, and describes the use of a polyester film whose melting point is from 210 to 245° C., whose planar orientation is from 0.10 to 0.16, whose shrinkage is smaller than or equal to 10%, and whose density is less than 1.385 g/cm3. The film features good formability and good adhesion to metal and is intended by way of example for use in lamination of cans. A disadvantage is that the polyesters used do not sufficiently reduce the strength and crystallinity of the film, the reduction in stiffness of the film therefore being insufficient.
EP-A-1 453 895 provides a further example and claims a biaxially oriented polyester film whose glass transition temperature is greater than or equal to 60° C. and whose density is smaller than or equal to 1.38 g/cm3 and whose dead-fold retainability is at least 60%. A disadvantage is that the polyesters used do not sufficiently reduce the strength of the film, the reduction in level of mechanical properties of the film therefore being insufficient.
EP-A-0 612 790 describes a transparent, biaxially oriented polyester film which comprises a particle system comprised of crosslinked polymers. The film is characterized by planar orientation Δp which is greater than 0.14. All of the films listed in the examples have planar orientation Δp of 0.17. By virtue of the planar orientation Δp of 0.17 cited in the examples, said films again cannot achieve the desired low mechanical strengths.
Heat-sealable, biaxially oriented polyester films are also known in the prior art.
EP-A-0 035 835 describes a heat-sealable polyester film to which particles are added in the sealable layer in order to improve winding performance, the average size of the particles exceeding the thickness of the sealable layer. The particulate materials added form surface protrusions which inhibit undesired blocking and adhesion of the film to rolls or guides. The selection of particles with diameter greater than the thickness of the sealable layer and with the concentrations cited in the examples impairs the seal performance of the film. The seal seam is produced at 140° C. and seal seam strength is determined at 23° C., and is in the range from 63 to 120 N/m (corresponding to from 0.97 to 1.8 N/15 mm of film width). The biaxially oriented polyester film does not have the low mechanical strength demanded with the desired feel and with good formability.
EP-A-0 515 096 describes a heat-sealable polyester film which has an additional additive on the sealable layer. The additive can by way of example comprise inorganic particles and is preferably applied in the form of an aqueous layer to the film during its production. The intention is thereby to retain the good sealing properties while the film has good processability. Said specification does not cite the sealing temperature range of the film. The seal seam is produced at 140° C. and seal seam strength is determined at 23° C., and is more than 200 N/m (corresponding to 3 N/15 mm of film width). Seal seam strength of 275 N/m (corresponding to 4.125 N/15 mm of film width) is cited for a sealable layer of thickness 3 μm. The biaxially oriented polyester film does not have the low mechanical strength demanded with the desired feel and with good formability.
EP-A-1 138 480 describes a heat-sealable polyester film with a base layer B, with a heat-sealable outer layer A, and with a further, non-sealable outer layer C. The minimum sealing temperature of the sealable outer layer A is at most 110° C. and its seal seam strength is at least 1.3 N/15 mm of film width. The topographies of the two outer layers A and C are characterized by certain features. The inventive film is particularly suitable for use in flexible packaging, and specifically and particularly for use on high-speed packaging machinery. The biaxially oriented polyester film does not have the low mechanical strength demanded with the desired feel and with good formability.